In order to be considered as suitable replacements for conventional film projectors, digital projection systems must meet demanding requirements for image quality. This is particularly true for cinematic projection systems. In order to provide a competitive alternative to conventional cinematic-quality projectors, digital projection apparatus must meet high standards of performance, providing high resolution, wide color gamut, high brightness, and frame-sequential contrast ratios exceeding 1,000:1.
The most promising solutions for digital cinema projection employ, as image forming devices, one of two types of spatial light modulators. The first type of spatial light modulator is the digital micromirror device (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DMD devices are described in a number of patents, for example U.S. Pat. Nos. 4,441,791; 5,535,047; 5,600,383 (all to Hornbeck); and U.S. Pat. No. 5,719,695 (Heimbuch). Optical designs for projection apparatus employing DMDs are disclosed in U.S. Patent No. 5,914,818 (Tejada et al.); U.S. Pat. No. 5,930,050 (Dewald); U.S. Pat. No. 6,008,951(Anderson); and U.S. Pat. No. 6,089,717 (Iwai). DMD-based projectors demonstrate some capability to provide the necessary light throughput, contrast ratio, and color gamut, however, inherent resolution limitations (with current devices providing only 1024×768 pixels) and high component and system costs have restricted DMD acceptability for high-quality digital cinema projection.
The second type of spatial light modulator used for digital projection is the liquid crystal device (LCD). The LCD forms an image as an array of pixels by selectively modulating the polarization state of incident light for each corresponding pixel. LCDs appear to have advantages as spatial light modulators for high-quality digital cinema projection systems. Among examples of electronic projection apparatus that utilize LCD spatial light modulators are those disclosed in U.S. Pat. No. 5,808,795 (Shimomura et al.); U.S. Pat. No. 5,798,819 (Hattori et al.); U.S. Pat. No. 5,918,961 (Ueda); U.S. Pat. No. 6,010,221 (Maki et al.); and U.S. Pat. No. 6,062,694 (Oikawa et al.). Recently, Eastman Kodak Company and JVC demonstrated a LCD-based projector capable of high-resolution (2,000×1,280 pixels), high frame sequential contrast (in excess of 1,000:1), and high light throughput (up to 12,000 lumens). This system utilizes three vertically aligned LCDs (one per color) driven via silicon backplane electronics.
JVC and others have developed vertically aligned LCDs, which are addressed via a silicon backplane. The JVC LCD devices are described, in part, in U.S. Pat. No. 5,570,213 (Ruiz et al.) and U.S. Pat. No. 5,620,755 (Smith, Jr. et al.). In contrast to early twisted nematic or cholesteric LCDs, vertically aligned LCDs promise to provide much higher modulation contrast ratios (in excess of 2,000:1). It is instructive to note that, in order to obtain on-screen frame sequential contrast of 1,000:1 or better, the entire system must produce greater than 1,000:1 contrast, and both the LCDs and any necessary internal polarization optics must each separately provide ˜2,000:1 contrast. Among considerations for contrast are variables such as spectral bandwidth and angular width of incident light, expressed as an f/# value. Contrast tends to decrease as spectral bandwidth increases and as the f/# decreases. Modulation contrast of LCD components is also reduced by residual de-polarization or misoriented polarization, such as by thermally induced stress birefringence.
Thus, as is known to those skilled in the digital projection art, the optical performance provided by LCD based electronic projection system is, in large part, defined by the characteristics of the LCDs themselves and by the polarization optics that support LCD projection. The performance of polarization separation optics, such as polarization beamsplitters, pre-polarizers, and polarizer/analyzer components, is of particular importance for obtaining high contrast ratios.
The most common conventional polarization beamsplitter solution, which is used in many projection systems, is the traditional MacNeille prism, disclosed in U.S. Pat. No. 2,403,731. This device has been shown to provide a good extinction ratio (on the order of 300:1). However, this standard prism operates well only with incident light over a limited range of angles (a few degrees), and in operation it can experience fabrication or thermally induced stress, which is realized as stress birefringence and loss of image contrast.
Recognizing some of the problems inherent to MacNeille prism use, alternative polarization beamsplitter technologies have been proposed to meet the needs of an LCD based digital cinema projection system. For example, the beamsplitter disclosed in U.S. Pat. No. 5,912,762 (Li et al.) comprises a plurality of thin film layers sandwiched between two dove prisms and attempts to achieve high extinction ratios for both polarization states. Other projector designs have employed liquid-immersion polarization beamsplitters. However, neither of these alternate solutions is ideal, as these designs are affected by fabrication issues, performance limits, and cost concerns.
Wire grid polarizers have been in existence for a number of years, and were initially used in radio-frequency applications and in optical applications using non-visible light sources. Until recently, use of wire grid polarizers with light in the visible spectrum has been limited, largely due to constraints of device performance or manufacture. However, as is disclosed in U.S. Pat. No. 6,122,103 (Perkins et al.), higher quality wire grid polarizers and beamsplitters have now been developed for broadband use in the visible spectrum. Among these are new devices commercially available from Moxtek Inc. of Orem, Utah. While existing wire grid polarizers, including the devices described in U.S. Pat. No. 6,122,103, may not exhibit all of the necessary performance characteristics needed for obtaining the high contrast required for digital cinema projection, these devices do have a number of advantages. When compared against standard polarizers, wire grid polarization devices exhibit relatively high extinction ratios and high efficiency. Additionally, the contrast performance of these wire grid devices also has broader angular acceptance (NA or numerical aperture) and more robust thermal performance (with less opportunity for thermally induced stress birefringence) than standard polarization devices. Furthermore, the wire grid polarizers are robust relative to harsh environmental conditions, such as light intensity, temperature, and vibration. These devices perform well under conditions of different color channels, with the exception that response within the blue light channel may require additional compensation.
Wire grid polarizing beamsplitter (PBS) devices have been employed in some digital projection apparatus, with some degree of success. For example, U.S. Pat. No. 6,243,199 (Hansen et al.) discloses use of a broadband wire grid polarizing beamsplitter for projection display applications. U.S. Pat. Nos. 6,234,634 and 6,447,120 (both to Hansen et al.) disclose a wire grid polarizing beamsplitter that functions as both polarizer and analyzer in a digital image projection system. U.S. Pat. No. 6,234,634 states that very low effective f/#s can be achieved using wire grid PBS, with some loss of contrast, however. Notably, neither U.S. Pat. No. 6,234,634 nor U.S. Pat. No. 6,447,120 makes mention of the use of a polarization compensator for correction of light leakage. However, U.S. Pat. No. 6,585,378 (Kurtz et al.), which is assigned to the same assignee as the present invention, discloses an optical system employing both wire grid polarizers and LCDs, which is further complemented by a polarization compensator.
Of particular interest and relevance for the apparatus and methods of the present invention, it must be emphasized that neither the wire grid polarizer, nor the wire grid polarization beamsplitter, provide the target polarization extinction ratio performance (nominally >2,000:1) needed to achieve the desired projection system frame sequential contrast of 1,000:1 or better. Individually, both of these components provide less than 1,000:1 contrast under best conditions. Performance falls off further in the blue spectrum. Finally, the problems of designing an optimized configuration of polarization optics, including wire grid polarizers, in combination with the LCDs, color optics, and projection lens, have not been addressed either for electronic projection in general, or for digital cinema projection in particular.
There have been a number of conventional methods proposed for increasing contrast and eliminating birefringence effects when using LCDs. For example, conventional methods include use of a separate polarizer/analyzer combination with the LCD, sometimes with an additional compensator as in U.S. Pat. No. 5,298,199 (Hirose et al.) which discloses use of a biaxial film compensator for optical birefringence of the LCD. Similarly, U.S. Pat. No. 4,701,028 (Clerc et al.) discloses birefringence compensation built into the structure of the LCD itself. U.S. Pat. No. 5,039,185 (Uchida et al.) discloses a homeotropic LCD with compensator provided between a polarizer/analyzer pair. For projector apparatus using an LCD with the conventional MacNeille prism type polarization beamsplitter, a ¼ waveplate used as a compensator has been disclosed, as in U.S. Pat. No. 5,576,854 (Schmidt et al.), which also discloses use of additional phase retardation as compensation for inherent LCD birefringent effects.
Without compensation, the polarization beamsplitter provides acceptable contrast when incident light is within a low numerical aperture. However, in order to achieve high brightness levels, it is most advantages for an optical system to have a high numerical aperture, so that it is able to gather incident light at larger oblique angles. The conflicting goals of maintaining high brightness and high contrast ratio present a significant design problem for polarization components. Light leakage in the OFF state must be minimal in order to achieve high contrast levels. Yet, light leakage is most pronounced for incident light at the oblique angles required for achieving high brightness.
Compensator requirements for wire grid polarizing beamsplitter devices differ significantly from more conventional use of compensators with polarizing beamsplitter devices based on the MacNeille prism design as was noted in reference to U.S. Pat. No. 5,576,854. For example, performance results indicate that the use of a ¼ waveplate, a conventional approach when using the MacNeille prism, is not a suitable solution and can even degrade contrast ratio when used in combination with a wire grid polarizing beamsplitter.
A number of problems must be solved when using compensator components in a digital cinema projection system that employs LCD spatial light modulators with wire grid polarization components. The need for compact packaging of optical components in a digital projection apparatus introduces space constraints that can limit the number of options available for positioning a compensator. This physical constraint is made even more demanding for optical systems that use a low f/#. As has been noted hereinabove, low f/# systems are advantaged for achieving higher overall luminance. It is also desirable to allow some degree of adjustability for compensator components in a digital projector design. Allowing adjustability, however, tends to make packaging requirements even more complex. As another consideration, there is also a need to protect the LCD and other components from ambient dust and dirt, which would degrade image quality and overall device performance.
Prior art solutions for mounting an LCD with its associated polarization and compensator components provide some useful results, but fall somewhat short of the mark in handling the above-mentioned problems. For example:                U.S. Pat. No. 5,576,854 (Schmidt et al.) discloses an adjustable compensator plate disposed between a polarization plate and a liquid crystal light valve to compensate for polarization irregularities in the cone of light projected onto the LCD modulator;        U.S. Pat. No. 6,460,998 (Watanabe) discloses a polarizer having an angle adjustment mechanism in which a first frame arranged in the vicinity of an electrooptic device pivotally supports a second frame having a polarizer thereon, allowing a slight adjustment of the polarizer about a pivot point, where the pivot point is optically off-axis;        U.S. Pat. No. 6,280,036 (Suzuki) discloses a dust preventive structure for spatial light modulator components within a digital projection apparatus, with a field lens attached to each LCD device;        U.S. Pat. No. 5,743,611 (Yamaguchi et al.) discloses dust protection by hermetically sealed components about an LCD;        U.S. Pat. No. 6,375,328 (Hashizume et al.) discloses transparent cover plates bonded to the surface of LCD spatial light modulators; and        U.S. Pat. No. 6,414,734 (Shigeta et al.) discloses a sealed unit that houses an LCD with support components.        
Significantly, each of the above solutions fails to meet one or more important requirements for compactness and suitability for a low f/# system, precision adjustability of compensator retardance, and protection of the LCD from dust and dirt contamination. Thus, none of these solutions is ideally suited for mounting a compact adjustable compensator for a low f/# optical system in an electronic projection apparatus that comprises an LCD with a wire grid polarization beamsplitter. For example, the solution of U.S. Pat. No. 5,576,854, while it provides adjustability, is not suitable for a system using low f/# optics and does not provide dust protection. The solution of U.S. Pat. No. 6,460,998 is intended only to provide a very limited adjustability of a polarizer component (not a compensator) over a few degrees, where this component is pivoted in an off-axis manner. This solution could be used in a low f/# system; however, there is no consideration for dust protection noted in U.S. Pat. No. 6,460,998. U.S. Pat. No. 6,280,036 provides only a lens and seal against the surface of an LCD, with no accommodation for an adjustable compensator. Similarly, U.S. Pat. Nos. 5,743,611 and 6,375,328 provide dust protection for the LCD, but with no adjustability for polarization or compensator components. U.S. Pat. No. 6,414,734 provides a possible method for sealing any number of support components with an LCD, which could be useful in a low f/# system and is relatively dust-free, but does not provide any adjustability of components.
In conventional LCD component packaging, the liquid crystal light modulator itself is provided within a frame and protected by a glass cover plate. As is well known in the optical arts, a cover plate is detrimental to the optical path, often compromising optical performance. Depending on the application, a glass plate in the optical path introduces the potential for various undesirable optical effects, aberrations, and unwanted reflection or refraction of incident or stray light. However, some type of protection from dust and dirt must be provided to the LCD, even with the potential disadvantages of a glass cover plate.
Thus, it can be seen that there is a need for a mounting arrangement of a controllably adjustable compensator for an LCD spatial light modulator, where this mounting arrangement is suitable for a low f/# optics system and also provides protection from dust and dirt with a minimum number of components.